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Seth Rich, a young Democratic National Convention staffer, was walking home in an up-market Washington neighbourhood, talking to his girlfriend on the phone when he was shot dead.

That's as much as anyone knows for sure about the death of the 27-year-old. Why he was killed and who was responsible is now at the centre of political intrigue and conspiracy theories leading all the way to presidential candidate Hillary Clinton.

WikiLeaks founder Julian Assange has suggested Rich was gunned down in Washington DC for being the "source" of a damaging leak that threw last month's Democratic National Convention into chaos.

The so-called #DNCLeaks, a trove of 20,000 emails stolen from the DNC servers, contained explosive details of collusion between the Clinton camp and the DNC to undermine the Bernie Sanders campaign, as well as a cosy relationship between the DNC and media organisations including CNN, Politico, the Wall Street Journal and the Washington Post.

The emails led to the resignation of DNC chairman Debbie Wasserman Schultz and a number of senior staffers on the eve of the convention, which was marred by protests, disruptions and walkouts by Sanders-supporting delegates.

ANNOUNCE: WikiLeaks has decided to issue a US$20k reward for information leading to conviction for the murder of DNC staffer Seth Rich.

Rich, 27, who worked as an analyst with the DNC, was shot several times in the back around 4am on July 10 while walking home to his apartment in the upscale suburb of Bloomingdale.

Police said they believed the motive was robbery, and there was no evidence his murder was connected to his work.

However, Rich's father Joel Rich said it did not appear to be a robbery. "If it was a robbery, it failed, because he still had his watch, he still had his money, he still had his credit cards, still had his phone, so it was a wasted effort except we lost a life," he told a local TV station.

His mother Mary said they "took his life for literally no reason". "There had been a struggle. His hands were bruised, his knees are bruised, his face is bruised, and yet he had two shots to his back, and yet they never took anything," she said.

Rich, who had been on the phone to his girlfriend when he was shot, was still conscious and breathing when found by police but later died in hospital. Police have no witnesses and no suspects, and are offering a $US25,000 reward for information.

On Tuesday, WikiLeaks announced it was offering its own $US20,000 reward for information leading to the arrest of Rich's killer, sparking widespread speculation that he was the source of the leaks, which had previously been pinned on Russia.

Speaking on Dutch television later that day, Assange cited Rich's death as an example of the risks whistleblowers take, while refusing to confirm Rich was the source of the leaks.

"Whistleblowers go to significant efforts to get us material and often very significant risks," Assange said. "A 27-year-old, works for the DNC, was shot in the back, murdered just a few weeks ago for unknown reasons as he was walking down the street in Washington."

When the host replied that it was a robbery, Assange replied: "No. There's no finding. I'm suggesting that our sources take risks, and they become concerned to see things occurring like that."

Pressed to confirm Rich was the source, Assange said: "We don't comment on who our sources are. We have to understand how high the stakes are in the United States. Our sources face serious risks, that's why they come to us, so we can protect their anonymity.

"Others have suggested that [Rich was murdered for being the source]. We are investigating to understand what happened in that situation with Seth Rich. I think it is a concerning situation. There's not a conclusion yet, we wouldn't be willing to state a conclusion, but we are concerned about it.

"More importantly, a variety of WikiLeaks sources are concerned when that kind of thing happens."

However, family spokesman Brad Bauman, speaking on behalf of Rich's parents, told The Daily Mail the comments were not helpful.

"Some are attempting to politicise this horrible tragedy, and in their attempts to do so, are actually causing more harm than good and impeding on the ability for law enforcement to properly do their job," he said.

Even before Assange's comments, conspiracy theories had been swirling around Rich's death and his name added to lists of people connected to the Clintons who have died.

Websites are such as What Really Happened and Clinton Memorial Library obsessively catalogue deaths going back decades.

"I can't give a guarantee that everyone on my list was killed deliberately but they all have the potential," Michael Rivero, webmaster of What Really Happened, told The Daily Mail.

"But by the general law of statistics there are just way too many of them."